Column laboratory experiments were
employed to assess the leaching behaviour of pyroclastic
glassy ash deposits collected in the central
Main Ethiopian Rift, where surface and groundwater
resources are affected by fluoride (F−) pollution,
which is the cause of an endemic disease (fluorosis)
in the local community. To elucidate the source of F−
and simulate the water–rock interaction processes, as
well as quantify its distribution within different grain
sizes, the pyroclastic ash was analysed by XRF, XRD
and SEM and separated into coarse and fine fractions.
Three columns were filled with raw (unsieved),
coarse (63 μm–2 mm) and fine (<63 μm) fraction,
respectively, and flushed with synthetic rain water in
saturated conditions. Very fast F− leaching was
observed in the fine fraction column at the start of
the experiment, while in the other two columns, F−
was slowly released; in addition, a strong accumulation
of F− was found in the fine fraction. The effect
was more pronounced in the fine fraction column due
to the available effective adsorbing surface area.
Subsequent to elution experiments, the columns were
characterised via moment analysis of tracer test.
Finally, flow and transport modelling (MODFLOW-
2000 and MT3DMS) was employed to compute the
amount of F− adsorbed onto the solid phase, comparing
the calculated conservative transport of F− and the
observed concentrations. The results of this study
suggest that fluoride is a fundamental constituent of
the glass phase (about 0.3 wt.%) and that it is released
during the incongruent dissolution of glassy particles.
Dissolution of coatings on glass particles could
provide an additional contribution to the geochemistry
of the interacting fluids. These processes are more
effective in the fine fraction due to a much higher
reactive (specific) surface area.

Column laboratory experiments were
employed to assess the leaching behaviour of pyroclastic
glassy ash deposits collected in the central
Main Ethiopian Rift, where surface and groundwater
resources are affected by fluoride (F−) pollution,
which is the cause of an endemic disease (fluorosis)
in the local community. To elucidate the source of F−
and simulate the water–rock interaction processes, as
well as quantify its distribution within different grain
sizes, the pyroclastic ash was analysed by XRF, XRD
and SEM and separated into coarse and fine fractions.
Three columns were filled with raw (unsieved),
coarse (63 μm–2 mm) and fine (<63 μm) fraction,
respectively, and flushed with synthetic rain water in
saturated conditions. Very fast F− leaching was
observed in the fine fraction column at the start of
the experiment, while in the other two columns, F−
was slowly released; in addition, a strong accumulation
of F− was found in the fine fraction. The effect
was more pronounced in the fine fraction column due
to the available effective adsorbing surface area.
Subsequent to elution experiments, the columns were
characterised via moment analysis of tracer test.
Finally, flow and transport modelling (MODFLOW-
2000 and MT3DMS) was employed to compute the
amount of F− adsorbed onto the solid phase, comparing
the calculated conservative transport of F− and the
observed concentrations. The results of this study
suggest that fluoride is a fundamental constituent of
the glass phase (about 0.3 wt.%) and that it is released
during the incongruent dissolution of glassy particles.
Dissolution of coatings on glass particles could
provide an additional contribution to the geochemistry
of the interacting fluids. These processes are more
effective in the fine fraction due to a much higher
reactive (specific) surface area.